Promise.if implementation

Promise.if.js

/**
 * Resolve or reject a {@link Promise|Promise} based on the return value of a function.
 * If a truthy value is returned, then the {@link Promise|Promise} is
 * resolved with the truthy value; otherwise, if a falsy value, it's rejected with the reason
 * defined as an instance of Error
 *
 * @memberof Promise
 * @param {Function} fn Function to invoke and compare the return value of
 * @param {*} [comparison] Optional comparison value to compare with the return value of the function;
 * otherwise, default to truthy/falsy comparison
 * @param {*} [initial] Optional initial value to pass to the function
 * @returns {Promise} A {@link Promise|Promise} that is either resolved with the function's return value;
 * otherwise, it's rejected with the reason defined as an instance of Error
 */
Promise.if = (fn, comparison, initial) => {
    const hasComparisonArg = comparison !== undefined;

    return new Promise((resolve, reject) => {
        const result = fn(initial);
        const isTruthy = hasComparisonArg ? comparison === result : Boolean(result);
        if (isTruthy) {
            resolve(result);
        } else {
            const err = new Error(`Invalid Promise if outcome, result: "${result}"${hasComparisonArg ? `, comparison: "${comparison}"` : ''}`);
            reject(err);
        }
    });
};

/**
 * Resolve or reject a {@link Promise|Promise} based on the return value of a function.
 * If a truthy value is returned, then the {@link Promise|Promise} is
 * resolved with the truthy value; otherwise, if a falsy value, it's rejected with the
 * reason defined as an instance of Error.
 *
 * @memberof Promise
 * @param {Function} fn Function to invoke and compare the return value of
 * @param {*} [comparison] Optional comparison value to compare with the return value of the function;
 * otherwise, default to truthy/falsy comparison
 * @returns {Promise} A {@link Promise|Promise} that is either resolved with the function's return value;
 * otherwise, it's rejected with the reason defined as an instance of Error
 */
Promise.prototype.if = function (fn, comparison) {
    return this.then(value => Promise.if(fn, comparison, value));
};

// Example(s)

Promise.if(() => 1092)
    .then(result => {
        console.log('Success::', result);
    })
    .catch(err => {
        console.error('Error::', err);
    });

Promise.resolve(Math.floor(Math.random() * 2000))
    .if(value => {
        console.log('Random number:', value);
        return value % 2 === 0;
    })
    .then(result => {
        console.log('Success::', result);
    })
    .catch(err => {
        console.error('Error::', err);
    });